Mendel's Pea Plant Experiments
Gregor Mendel (1822–1884) crossed pea plants with contrasting traits and recorded the ratios of offspring. His results, analysed mathematically, led to three laws that underpin all of genetics. He chose pea plants because they have clear traits, a short life cycle, and can be self- or cross-pollinated precisely.
Mendel's Three Laws
Summary of the three fundamental laws of inheritance.
| Law | Statement | Example |
|---|---|---|
| Dominance | One allele masks the expression of the other | Tall (T) dominant over dwarf (t) — Tt is tall |
| Segregation | Allele pairs separate during gamete formation | Tt parent produces T and t gametes in equal numbers |
| Independent Assortment | Genes on different chromosomes are inherited independently | Seed colour and seed shape are inherited independently |
Punnett Squares
A Punnett square predicts the possible genotypes and their ratios among offspring. For a monohybrid cross Aa × Aa: the genotypic ratio is 1 AA : 2 Aa : 1 aa, giving a 3:1 phenotypic ratio (dominant:recessive). For a dihybrid cross AaBb × AaBb, the phenotypic ratio is 9:3:3:1.
Beyond Mendel: Codominance and Sex Linkage
ABO blood groups show codominance (I^A and I^B are both fully expressed). Haemophilia and colour blindness are X-linked recessive — they appear more in males (XY) who inherit only one X chromosome and cannot compensate with a dominant allele.
HSC Exam Focus
Always draw a Punnett square in full — list parental genotypes, gametes, and all offspring genotypes. State ratios as 'genotypic' or 'phenotypic'. Practise sex-linked crosses using X^H X^h × X^H Y notation.
InstaTest
InstaTest: Genetics
5-question MCQ sprint covering Mendelian ratios, codominance, and sex-linked inheritance.